Tokkai Sho 62-270855 published by the Japanese Patent Office in 1987 discloses a prior art for suppressing heat generation of a V-belt. The controller of the prior art comprises a non-contact temperature sensor at a predetermined position between a drive pulley and a driven pulley, and the non-contact temperature sensor detects the radiant heat from the V-belt. When a state where the temperature of the V-belt exceeds a predetermined limiting temperature continues for a predetermined time, the controller controls a speed-reduction ratio to suppress heat generation by the V-belt.